Azeo-extractive distillation



Sept. 2, 1952 J. H. BoYD AZEO-EXTRACTIVE DISTILLATION 2 SHEETS-SHEET l Filed July 30, 1948 sept. 2, 1952 J. H. BOYD Filed July 50, 1948 2 SHEETS-SHEET 2 ATTORNEYS Patented Sept. 2, `1952 UNITED STATES P./-i'11=.1\1T OFFICE i James H.. Boyd, New York, N. Y., assigner to Phillips Petroleum Company, acorporation of Delaware Application July so, 194s, srifnNo.41,454.i

' e claims. (ci. 2oz-39.5)

This invention relates generally to the separation of -dicultly separable hydrocarbons. In one aspect this invention relates to the separation of close-boiling aliphatic hydrocarbons and alicyclic hydrocarbon by distillation. In a moreY particular aspect this invention relates to the separation of close-boiling isoparatnic hydrocarbons and naphthenic hydrocarbons by distillation. This invention is particularly useful for the separation of neohexane and cyclopentane by distilling a mixture of these hydrocarbonsin the presence of "an azeotrope-forming solvent and an extractive distillation solvent. f

Azeotropic and extractive distillations forY the separation of close-boiling hydrocarbons, not separable by conventional distillation procedures, are commercial operations of importance. Both methods of distillation have the property in commonthat a substance, not present in the mixture to be separated, is added to increase the diierence in volatility of theV diiicultly separable components of the mixture. Azeotropic distillation may be dened as a process in which the substance added forms an -azeotrope with one or more of the components and `by virtue of'this fact is present on most of the plates of the'distillation or fractionation column in appreciable concentration. Extractive distillation may be clened as a distillation Yin the presence of a substance that is relatively non-volatile compared to the components to be separated and which, therefore,` is charged continuously near the top of the distilling or fractionating columnv so that an appreciable concentration of solvent is maintained on all the plates of the column.

-The separation of close-boiling hydrocarbon mixtures by azeotropic distillation depends upon the addition to the hydrocarbon mixture of a material, sometimes loosely referred to as a solvent,`that forms a minimum boiling point azeotrope With one of the hydrocarbons in the mixture;. some of the solvent is maintained in 'the fractionator bottomfat least until all of the hydrocarbon that forms the azeotrope With the solvent has been removed from the system. The eiiect'of formation of the azeotrope is to increase significantly the volatility oi the hydrocarbon containedV in the azeotrope relative to the volatilities of the other hydrocarbons in the mixture, thus permitting separations that cannot be made by using the-'ordinary Vor normal volatilities of the hydrocarbons. The following are examples of azeotropic-formers or entrainers thatare used to separate i difcultly separable hydrocarbons: methyl" formate `(b'oiling'point 31.5- C), methanolY (boiling point 64.5 CL), acetone (boiling point 56.5 `C.), ethanol (boiling point 78"C.`).f methyl' cyanide(boiling point 82uv CJ, acetic acid (boiling point 118 C.), ethylene glycol mono methyl ether (boiling point-125 CJ."

In extractve distillation a relativelyhigliboiling polar solvent is added to the fractionatin'g or distillation column near the top, andcln descending it scrubs the ascending vapors;V "f This solvent, if properly selected, depresses the inormal volatility of one of the hydrocarbonsin-th'e mixture morethan the volatilities ofthe other hydrocarbons in the mixture, thus providing. an abnormally great difference in theeffective'vola; tilities and thus facilitating separation by distillation. This derangement of the normal:volatilities of the hydrocarbons permits the separa-l tion of hydrocarbon mixtures which arevnormally inseparable by straight fractionation. The use of furfural inthe separation of normal'butane from butenes and in the separationof butenel from butadiene, butene-2 and vinyl acetyleneis now practiced in industrial operations. Solvents useful in extractive'distillations are generally polar and less volatile than the least volatile hydrocarbon in the mixture to be separated. Such solvents include furfural (boiling point 161 C.), phenol (boiling point 183 C.),'benz`onitr`ile (boiling point 191 C.), nitrobenzene (boiling point 210 C.),- dimethyl formamide (boiling point 153- (31),v and methyl levulinate -(boiling point 196 0.).

Thus the azeotropic separation of h-ydrocarbons in rdiiiicultly separable hydrocarbon mixture depends on increasing the relativevolatility of one or more of the hydrocarbons to beseplarated by addition of the azeotrope forming solvent to the distillation system. Hydrocarbon separation by extractive distillation depends -upon the lowering of the volatility of one or more of the hydrocarbons in thev mixture by the addition of a relatively non-volatile polar solvent to the distillation system.

In spite of the phenomenonvknow'n a's'azeotropic distillation, there are instances Where such distillation of petroleum fractions is not as ef' fective as desired, and considerable recycling is necessary, either internally in the column'by the use of very high reiiuxes or externallyby repetition of distillation of the partially separated products.` The separationlof cyclopentane (boiling point 49.26 C.) and neohexane (boiling point 49.75 C.) is difficult even when using methyl formate as an `entrainer orazeotropeformer for preferential removalof the nohexane of this invention is to provide an improved method for the resolution of a mixture of close-boiling hydrocarbons by combining the two processes of azeotropic and extractve V.distillation into one.

distillation process. It is a further object of this invention to resolve a mixture yof close-boiling;

aliphatic and alicyclic hydrocarbons by distilling the mixture in the presence of an azeotropetillinegfthe ,mixture Ain.the presence. oimethyl formate; and furfural. Still another Vobject Aof this inventionv is to resolve laymixture of isoheptane and cyclohexane .by distilling the mixture in thepresenoe of `methanol and nitrobenzene. Furtherandadditional objectsvvillbe apparent from my :.disclosure.

I have found that mixtures of close-boiling hydrocarbonsthat can not be readily separatedby ordinary distillation;procedures, Acan be resolved by, a.iclistillationyprocess- Which is a combination ofl .azeotropic .and extractive distillation. In accordancewith my invention I effect the distillaton:1of;;mixtures .of ydifcultly separable hydrocarbone inthe presence oa-solvent that forms 4an: azeotrope With at least one vof the hydrocarbon components of the mixture and in the Vpresencerof ran-extractivesolvent thatdepresses the volatility of at leastY one v.ofthe other hydrocarbon components of the mixture.;l By proper selection of,' an-azeotrope-forming 'solvent and of an -extraotivedistillation solvent the .diierence between the relative'.volatilities, ofthe. one or more hydrocarbons in the overhead elluent fromthe distillation 4andof the oneor more hydrocarbons being withdrawnas the bottoms product from the distillation can-be heightened by the opposite effects ,oi-the twogsolvents en the relative volatilities of thehydrocarbons in thesystem This kind of distillation may be termed azeoextractive dis: tillation'.:

lBroadly..my nventomcontempates.the Sonar rat.on..-of close-boiling. hydrocarbons. by azcoextractive distillation: In one or its aspects my .inventionicontemplates the separationof closef boliilgfaliphatic and-alicyclic hydrocarbons by azeo-extractive distillation. lilly-invention is particularly-valuable; for the separationof mixtures oigneohexane andcyclopentane and of at least -one -of the isoheptanes and cyclohexane, by azeo- 'miscibleror immiscible in eachother, for-use in separating ,the components of a given close-boiling-hydrocarbongfracton, the morevolatile soll vent, added mustform anazeotrope with at least one of the hydrocarbonfcomponents of the mixture-to be separated.- It also should boil Well .below the higher boiling solvent added and here .af-.temperature interval betweenjhe normal boiling points of 20-40" C. is desirable. It is also desirable that the azeotrope-forming solvent shall be Water soluble or not completely miscible with the hydrocarbon in the azeotrope at temperatures 20 C. below the temperature of condensation of the azeotrope. This is to facilitate separation of the azeotropic solvent from the hydrocarbon removedinthe azeotrope.; These.. properties are desirablein saidfsolventfl The heavier solvent shall be higher-boiling than the highest boiling hydrocarbon in the mixture. Ordinarily the heavy solvent will be separated from the hydrocarbon-solvent mixture Withdrawn as bottoms from the distillation apparatus by a second distillation-step though selective solvent extraction or fractional crystallization can be employed When the properties of the hydrocarbons and the solvent selected make either process advantageous.

The diagrammatic drawings represent merely preferred embodiments .Of my invention, but they are useful forr completely understanding. my i121 vention. Figure 1 is `a flow diagram forpracticing my invention to separate ahydrocarbon mixture containingabout 1Q lper cent neohexaneand about per cent -Cylopentane using anazeotrope former or entrainerand `an extractive solvent which are immiscible in each othen The hydrocarbon mixture is introduced via line I into distillation column 2 which; is a columnof conventional design, such as avpacked or bubble-cap column, Vwherein rectiiication occurs.; The azen-'- trope-forming solvent orentrainer for the neohexane is introduced to the. distillation column eitherv via Vline 3 withthe hydrocarbon mixture or separatelywvia -line 4 for both. The selective solvent for depressing the ...Volatility of the cyclopentane is introduced to the iractionating column at or near the .topvia line 5 -inorder that it can Scrub the ascending vapors. The pverhead product from column 2 comprising neohexane vv and entrainer passes via line B to vseparator 'I where the entraineris separated from the n eohexane by any suitablemeans such as Washing -wth water Orthersoli/ entrv phase. ;separation, etc. Substantially pure neohexane is withdrawn from separator 1 vialine IIl,.-and the entrainer from separator- 1 isreturnedto col urnn 2 Vialines 8,

comprising Vcyclopentane,selective solvent. and, possiblyra small amount of entrainer passes via line I I to distillationvcolumn I2. Aber distillation the overhead product from column I2 containing vcylopentagne and.ithe smal1 amount of Y entranenpasses vialine I l to separator I3;y Subst antially pure cyclopentane is withdrawn via line I5 and the entrainer. is returned to column 2 va linesv I6. 9 and 3, the bottoms product from column I2 Whichis substantially pure selective solvent is returned -to column 2 .via vlines IS .and 5. Fresh entrainer and selective solvent are introduced to the system vialines II and I8 respectively.

Alternatively, column2 may be operated so that substantially .all of the entrainer is taken overhead Vwith lthe neohexane.. In that event there Will be very little, if any, entrainer in lines I4 and I6 and the system can be operated without separator I3. Then, column I2 would be replaced by any suitable means forv separating Vtheselective solvent from the cyclopentane, and substantially pure cyolopentane would be obtained inline i4, and the selectiveV solvent Would be returned to col'- umn 2 vialines I9 and 5.

Lines 20 and 22 are placed inthe system for the .removal of entraner from'thesystem. andline 2l to remov'the'entrainer'and/orselective solvent from"the-system via thek lines-provided therefor and to" separateY the 'contaminants in purication means not shown" to prevent 'the' Va'zcumulation oi 'these `undesirable materials-in the system. f r rAnother` embodiment vof my-Qinvention kis shown by Figure 2 whereina hydrocarbonmix'- tu'r'e of ne'ohexane andcyclopentane is separated byl azeo-eX-tractive distillation -using tvvoemiscible solvents, vrsuch'as Vmethyl 'formatef-asfthefazeoi, trope-former vvfor ent-rainer, V`a'nol furffuralgfas the selective solvent; In' this embodiment-the miscibility ofc-the entrainer-.and selective solvent'lis su'ici'ently high that the selective solvent ca'n ex'- tractlthe entrainer from any of-v the hydrocarbons in sthemixture to beseparated; "In accordance With'my invention, ahydrocarbon mixture ofcy' clopentane'and neohexane is introduced vvia line 3j to fractionation or 'distillation `columnl32 which maybe-,similar tocolumn..2"inFigure k"The methyl formate enters column32V via'1ine33gand,- ifv desired, V`additionaly methyl formate' .is added to ,the lower portion of the column Vialine 34. lFur-- fural', V'the selective? solvent; saturated or nearly saturated withmethyl formate :is introduced to column 32 at orznear the top tray yia .line 35. The overhead product from column32'contain4 ing-an azeotrope of neohexane and methyl formate passes vialine 35.*to liquid=liquidextractor 3,8;fforthe removalsof. the methyl formate: The

solution is returned to column 32 via line `35i, and

the remainderwispassed Via line 44 .to distillation column 45 Where the methyl formate is sepaf rated ,from the furfural. The methyl formate is thefoverhead product frompcolumn 4'5 and'itV is returnedvialines, 33 ,and 3l Vand/or 34to column 32. The .filrfuralis the. :bottoms product from column'llS, and .it passesvia lines 48'and'53 to VextractorV 46. If desired a portion of vthe furfural may be withdrawnxvialineflle for purification (no t shown). Extractorxdris similarsto 38, andA itis` used .tdeiect .liquid-liquid contact `of furfural, introducedvia line 53,with the mixture of methyl formate and cyclopentane, the overhead'pi'oduct. from column 31. A cyclopentane: rich phase'is withdrawn from 46 via line 5|, and the furfural p hase, containing dissolved methyl formate and V`cyclopentane, passes via `lines `52 andl`39 to 'column 31 along with. `the bottoms product from columnA '32; f `A portion of the methyl formate in line 41 may be passed via line 54 to line 35 to increase the methyl formate content of the furfural stream entering column 32. Fresh furfural and methyl formate are added via line 55 and 56 respectively, as required.

The specic conditions, such as temperature and pressure, thatre employee nypracticingfmy invention are dependent 'upo'n the hydrocarbons Ato be separated and upon the entraineria'nd selective solvent that are used. For most' separations,YV the'. fractionatir'ig columns will be operated at? approximately :atmospheric -pressure,2 but higher andllower pressures maybe used. `I"o`riexi ample, inlsome separation systems .it is necessary toem'ploy vacuum distillation procedures in order to prevent excessive decomposition of the selecf,`

tive'solvenu y :Referring tothesystem described in: Figure 2; When-operating at atmosphericv pressure', the. overheadztemperature in column 32 is about 30/ C: andszthe kettle .temperature varies from l 35 -to 45C. 'dependingupon the proportion of furfural in the kettle product. The kettle temperature in columnsr31 and 65 is-r162*C'.,lthe boilingpoint of furfural, and the overhead temperature of column .31 is between 26 land 49.3 C.,the boiling points of -the cyclopentane-methylformate azeotrope .and of cyclopentane respectively;-- vThe overhead temperature' offcolumnv 45 Vis 32 boilingpoint of methylformatafVH AReferring-to Figure'gl wherein. the entrainer and selective' solventare immiscible, the overhead temperature of -column 2 isV approximately-the boiling-point of the azeotrope ofneohexane and the entrainer,v and the kettle temperatureisloe-l tween-49.3 C., the boilingfpoint of-cyclopentane,

and the boiling'point of the selective solvent.- The overhead temperature ofl column I2 is about .49.324

C., andl thekettle temperature is-about the-boil-l ingpointofthe selective solvents' *if The advantages of practicing my inventionfare quite readilyapparentbfromfthepresent disclosuref- Byffcombining the opposite -4eiiectsof azeotropic .and extractive distillation -ina single separation step it is possible to'effect agiven separation with fewerftheoretical plates than When'using either method of: distillation'alone.v AlsoIrhave found that by combining azeotrcpic and extractive distillation a more complete sepa-` ration of hydrocarbonsvis obtained than=-when only one ofk the methods ofY distillation is'used. This fact is substantiated by the followingqexe amples.. v- .1

, Y VElifani-'ple I'. f `30 0 cc.` of.a mixtureh comprising approximately 50 volume per cent each of neohexane and ,cyc'lc'Jl-gpentaneV was clziargedV to the *kettle of.. a O .plate fractionating column Vvtogether. withjfan equal volume of methyly formatearid 20o ofrurfral; The; column. was, operated at vva Yreilux Vl'ftllQ. .0I -ldz'lgandmfurfural wasintroduced at a point just Ib'elc'vvvthe overhead talfr'egoi 'line at a rateof about. 50 C05 peijhpur-,- The .'verheadfnroc'i. which contained about.' 6,1volumefper cent neof hexaneY and 39 l olurne per cent methyl formate, was substantially free fof. cyclopentane.`

j l n l Eivamlle'll'ili column employed in Example vIV was Charge@ m5112355. C9? Oia .5.0-5.0 mixture einen# hfeldfcclpniene; 1225 ce off methyl formate. Reflux established and maintained fr; 3 hOllrS.-,Q',1l1ejcq1mfn was aeainoperaited ein '1`e1HX.ratiQ"Qf;151-1 @without theadditjin oflfurfural.' The overhead product contained' per cent methyl formate and 42 per cent hydrocarbon by volume.` The hydrocarbon, free of methyl formate, was found to consist of 66 volume per cent neohexane and 34 per cent cyclopentane.

From the above disclosure numerous variations SQQQQBSS vimini-thereon@ pfamyginventionyvill hesepper.- enttathoseskilled in theart. f- .Ilaifrn ST1-4Y l; in j 1-.7; The method of resolving-a hydrocarbon mixture .comprising cyclopcntone and neohexane whchfcomprises distilling said `mixture in the presence of methyl `formate to*y form an azeotrope with 'said neohexane and in the presence" of Ifurfur'al, to 'depress the volatility of said cyclopentane;zrecovering a. mixture of `methyl formate and neohexane substantially free: from cyclopentane as the overhead product of said distillation; 'separating :the mixture' of methyl formate and vneo-- hexane s'o as to recover neohexane by' contacting the`mixture @with -furfural and recovering fur'- furalfand cyclopentane as the vkettle Vproduct of said distillation.

2. `.The method of resolving a hydrocarboninixture-'comprising cyclopentane and neohexane Whichicomprises distilling said mixture inthe presence of vmethyl formate to form an aze'otrope with said'neohexane and inthe presence of furiural'to depress' the volatility 'of said cyclopen'- tane; recovering a mixture of methyl formate and neohexanesubstantially free from cyclopentaneas the overhead product of said distillation; separatingthe mixture of methyl formate and neo'hexane so as -to' recover neohexane by contacting the mixture -with furfural; recovering furfural and cyclopentane as the kettle product of vsaid* distillation; and separating said kettle product into its components by separately distilling the same so as to recover cyclopentane 'as an overhead fraction and furfural as a bottoms fraction?. 4,

3'. The method of resolving a hydrocarbon mixture comprising cyclopentane and neohexane which Acomprises distilling'said mixture `in the presence of methyl' formate to form an azeotro'pe with said neohexane and in the presence of furfural to depress the volatility of said -cyclopentane; recovering a mixture of methyl formate and n'eohexane substantially free from cyclopentane as the overhead product of said distillation; recovering a kettle product comprising cyclopentane, furfural, and methyl formate; distilling said kettle product so as to recover an overhead fraction comprising principally methyl formate and A cyclopentane; and contacting said lastnamedcverhead fraction withliquid furfural so F as to recover cyclopentan'e.

.y 4. .The method of resolving a hydrocarbon mixture comprising cyclopentane and neohexane WhChcomprise-s distilling said .mixture in the presence vof methyl formate which is introduced to the distillation .columnnear the bottom of said columnand in the presence of furfural which is substantially saturated with methyl. formate and `which is introduced .to the distillation column near the top oisaid column; withdrawing as .the overhead product of said distillation a mixture comprising neohexane and methyl formate substantiallyfree from cyclopentane; separating said overhead, product into its components by contacting same with furfural and recovering substantially pure neohexane; withdrawing as the bottoms product from said distillation a mixture comprising furfural, methyl formate, and cyclopentane; separating saidf'bottoms product into 8 itsv components; and Vrecoveringl agYcyclopente-liefh riehpimscfr u. l.- Q -@The method-"o, ,resolvinga: hydrocarbon mixture-comprising cyclopentaneand neohexane which. comprises distilling `said mixture .Vinkthe presence oirmethyl formatel which isintroduced to the1distillation column nearfthe bottom of said column and inthe presence of furfural which; is substantially saturated `with methyl formatewand which is introducedto the distillation-column Vnear the top orsadcolumnwithdrawingasfthe overhead product of said distillation amixtiue comprising Yneohexane and methyl formate substantially `free fromcyclofl pentane; withdrawing as the bottoms ,product from Y said distillation a mixture Comprising fur fural, methyl formate and; cyclopentane; dise tilling said bottoms productzina separate distilfI lationlstepso as to recover,- an overhead fraction comprising methyl formate# and cyclopentane; andgcontacting said last named overhead fracticnyvithfurfural so as to recover cyclopentane. 6i The method :of resolving a Vhydrocarbon mixture comprising cyclopentane and neohexane which comprises --distilling said mixture =in vthe presence of methyl formate which is introduced to the distillation column near the botto'm'ofsald column and 'inthe presencelof :furfural 'which is substantially saturated with methyl formate and which is introduced to 4the distillation column ;near the top of said column; withdrawing as the overhead' product of said :distillation a mixture comprising neohexane fand methyl formate substantially free from cyclopentane; separating said overhead product intoV vits components'by contacting same with fu'rfural and recovering substantially pure neohexane; f withdrawing Aas the bottoms product from said distillationa mixture comprising furfural.' methyl formate, and cyclopentane; distilling said bottoms product in a separate distillation step s0 as to recoverfan overhead traction comprising methyl formate and cyclopentane; and contacting said last named overhead fraction with furfural so as to recover cyclopentane.

JAMES H. BOYD.

REFERENCES CITED The following references are of record in the die of this patent:

UNITED STATES PATENTS OTHER REFERENCES Mair etal., Joumm of Research of the National Bureau `ol Standardsvol. 27,' Dages 39-.63 (July Griswold et al., Pure Hydrocarbons From Petroleum,4 38, Industrialand Engineering Chemistry, (i5-70,A (January 1946). .Y Y 

1. THE METHOD OF RESOLVING A HYDROCARBON MIXTURE COMPRISING CYCLOPENTONE AND NEOHEXANE WHICH COMPRISES DISTILLING SAID MIXTURE IN THE PRESENCE OF METHYL FORMATE TO FROM AN AZEOTROPE WITH SAID NEOHEXANE AND IN THE PRESENCE OF FURFURAL TO DEPRESS THE VOLATILLITY OF SAID CYCLOPENTANE; RECOVERING A MIXTURE OF METHYL FORMATE AND NEOHEXANE SUBSTANTIALLY FREE FRM CYCLOPENTANE AS THE OVERHEAD PRODUCT OF SAID DISTILLATION; SEPARATING THE MIXTURE OF METHYL FORMATE AND NEOHEXANE SO AS TO RECOVER NEOHEXANE BY CONTACTING THE MIXTURE WITH FURFURAL; AND RECOVERING FURFURAL AND CYCLOPENTANE AS THE KETTLE PRODUCT OF SAID DISTILLATION. 